Chromium is an unavoidable trace element of the raw material used in the manufacture of cement clinker. In the oxidizing and alkaline burning conditions of the cement kiln, hexavalent chromium (CrVI) may form. Hexavalent chromium is a powerful dermal irritant that is considered extremely toxic due to its high oxidation potential and ability to penetrate human tissue. It can cause skin sensitization, allergic reactions, and eczema. Chromium VI has high solubility in water and is released when cement is mixed with water. Thus, wet cement poses a health issue for workers who come into contact with wet cement or concrete.
Currently, the typical approach is to employ ferrous (iron II) sulfate to reduce hexavalent chromium (CrVI) to trivalent chromium (CrIII), which tends to precipitate from solution, thereby posing a much reduced risk as a dermal irritant.
Moreover, the use of ferrous sulfate is not efficient because the dosage amount required for reducing CrVI to CrIII is at least ten times the stoichiometric amount of ferrous sulfate otherwise required. The poor dosage efficiency is explained partly by the fact that ferrous sulfate readily oxidizes, from Iron (II) to Iron (III), during prolonged contact with air and water. This poor storage stability decreases the efficiency of ferrous sulfate, and can render it useless as a chromate reducer if not used quickly.
As mentioned in U.S. Pat. No. 4,784,691, the decrease in the ferrous (II) sulfate chromate-reducing ability may be avoided, or to some extent diminished, if the ferrous sulfate is used in the form of particles coated with an oxidation-preventing material. An inevitable disadvantage of this coating process, of course, is that added expense and further process steps are required. Also, it appears from Example 1 in the '691 patent that up to 30 times the stoichoimetric amount of ferrous sulfate is required.
Furthermore, problems arising from the use itself of ferrous sulfate are not necessarily avoided or diminished by further processing. Such problems include the likelihood that ferrous sulfate, particularly when used in amounts in excess of one half percent by weight cement, create an increased water demand and extended setting time in the cement. Moreover, when used in a dry powder form, ferrous sulfate is difficult to interblend uniformly within the cement, and this is another factor explaining its high dosage requirement.
A still further problem with ferrous sulfate is that its dosage efficiency decreases drastically at higher temperatures (e.g., 80° C. or above) and high humidity levels. This will often be the case when cement clinker is interground to produce cement. Because intergrinding is the preferred method for combining materials at a cement manufacturing plant, the ferrous sulfate will need to be incorporated at levels that are up to three times the amount typically required for interblending, thus making this method uneconomical.
In view of these concerns, novel methods and compositions are needed for reducing hexavalent chromium to trivalent chromium, particularly in cementitious compositions and materials.